Method and apparatus for measuring lithological characteristics of rocks

ABSTRACT

A method by which physical and mechanical characteristics of rocks are measured during drilling, comprises picking up a signal representing the vibrations of a train of rods forming part of drilling gear, selecting the components of the said signal which, after peak-clipping, are in a frequency band which is centered on, and preferably is related to, a characteristic frequency of the tool, establishing from the components thus selected, a value which is representative of the amplitude of the vibrations, and correlating this value with the drilling depth.

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[72] Inventors Claude Jean Quichaud T55] 7 References Cited Bills; UNITED STATES PATENTS g g ig bah 2,544,569 3/1951 Silverman 175 50 2,620,386 12/1952 Alspaugh et al. 175/50 211 Appl. No. 868,873

22 F1 d Oct 23 1969 2,654,244 10/1953 Bra1d 175/50 1 le 2,752,591 6/1956 Felbeck et a1. 175/50 [45] Patented Dec. 7, 1971 2,755,431 7/1956 Scherbatskoy 175/50 [73] Assrgnee Societe Anonyme dite: Societe Nationale des P i i 2,790,968 4/1957 Cook et al. 175/50 fig 'j j fi r 3,115,942 12/1963 Arps 175/50 Priorities Oct. 30,1968 3,520,375 7/1970 Raynal et al. 175/50 33 France Primary Examiner-James A. Leppink [3 1] 6905142; Attorney-Holcombe, Wetheril1& Brisebois Dec. 11, 1968, France, No. 177543; Feb.

27, 1969, France, No. 171873 ABSTRACT: A method by which physical and mechanical characteristics of rocks are measured during drilling, com- [54] METHOD AND APPARATUS FOR MEASURING prises picking up a signal representing the yibrations of a train LITHOLOGICAL CHA A OF ROCKS of rods formmg part of dr1ll1ng gear, selectmg the components 27 Claim 10 Drawing Figs, of the said signal which, after peak-clipping, are m a frequency 2 U s 7 band which is centered on, and preferably is related to, a [5 1 5/25, E characterisfic frequency of the tool establishing from the 5 1 "i 73/151 components thus selected, a value which is representative of l] l- ..I. th a pli ude of the vibrations and correlating value E211 49/00 .1116 drilling depth. [50] Field of Search 175/25, 24, 50; 73/151 PATENTEDBEB 7197: 3.626482 SMU l- UF 7 PATENTEU [15c Han 3.6263182 SHEET 5 0F 7 METHOD AND APPARATUS FOR MEASURING LITHOLOGICAL CHARACTERISTICS OF ROCKS The present inventionv relates to a method and apparatus for measuring lithological characteristics of rocks as they are being drilled and at the instant when the drilling tool acts on the rock.

Experiments have previously been made involving correlating the amplitude of the stresses observed in the upper part of the drilling rod with the mechanical characteristics of the rocks. The experiments have failed, because of the composite nature of the signal.

One of the objects of the invention is to provide means whereby it is possible to process the crude information delivered by a vibration pickup device, with the object of supplying instantaneous information relating to the characteristics of the rock as this is actually being drilled.

Another object of the invention is to provide means which will enable an instantaneous signal to be obtained which is representative of the physical and mechanical properties of the rock, as these are picked up by the drilling tool, and which signal is capable of being used after processing as a value from which command signals can be established for automatically controlling the drilling.

Another object of the invention is to provide means with which it is possible to deliver, as a function of the progress of the drilling, a diagraph which is representative of the physical and mechanical properties of the rocks encountered by the tool.

The method according to the invention makes it possible to obtain, as a function of the depth at which the tool is operating, a value which is directly related to the physical, mechanical and stratigraphic properties of the rocks being attached by the drilling tool.

The method according to the invention comprises collecting, at at least one point located on a measuring section of drilling gear which is provided at its lower part with a drilling tool, signals which are representative of the vibratory state of the drilling gear at this point, bringing the values of parasitic voltages which are beyond two predetermined levels and are of opposite signs, to values equal to these levels, selecting from the signal thus processed, a frequency band centered on a frequency equal to the product of the frequency of rotation of the tool times the number of operative elements of the tool, measuring the amplitude of the signal thus selected, the value thereof being directly related to the lithological properties of the rocks attached by the drilling tool, and correlating this value with the depth at which the tool is working.

During the course of the studies which led to the present invention, an unexpected efiecthas been noted, residing in the fact that the amplitude of the signal which is processed in order to eliminate parasitic components and which is selected in a frequency band centered on a frequency which is equal to the product of the frequency of rotation times the number of attacking elements of the tool, is representative of the lithological properties of the rocks.

The characteristic emission spectrum of the drilling tool, that is to say the spectrum of frequencies which arises when the attacking elements of the tool act on the rock for the purpose of breaking it down, varies with the frequency of rotation of the tool and with the number of attacking elements carried by the said tool. However, the spectrum of frequencies which is received is useless without filtration, because of the composite character which arises due to the transfer function of the rods.

On the other hand, filtration of this signal in the frequency band referred to above, enables a useful signal to be obtained if care is taken to eliminate parasitic signals.

According to one feature of the invention, in carrying out this method, the numbers which are characteristic of the arrangement of the operative elements of the tool are defined, in the case of cutter wheel tools, by the number of cutter wheels or by the number of teeth in one of the rows of teeth of a cutter wheel, and in the caselof diamond tools, by the number of diamond-bearing areas.

By way of example, in tools having cutter wheels, the number of wheels which form the primary attacking elements may be two, three or four, depending on circumstances.

In addition, the number of teeth carried by each cutter wheel can vary. There may be, for example, approximately 20 teeth in the outer row of teeth carried by the wheel and from eight to 10 teeth in the intermediate row of teeth carried by the wheel.

Similarly, in diamond tools, the number of elements for attacking the rock is determined by the number of surfaces on which the diamonds are mounted, these surfaces being separated by channels.

The method according to the invention can be carried into effect by collecting a single vibratory signal and processing it by the method referred to above.

The result thus obtained enables information to be collected regarding the lithological properties of good quality rocks.

According to one preferred mode of carrying the invention into effect, signals representing the vibratory state of the drilling gear are collected at at least one pair of points diametrically opposite one another on the drilling gear and the algebraic summation or the instantaneous differencing of these signals is effected.

According to a preferred mode of carrying the invention into effect, two vibratory signals are collected by means of pickup devices positioned on two diametrically opposed generatrices of the drilling gear, the two pickup devices being offset axially by from 2 centimeters to 9 meters.

During the course of tests it has been found possible to effect an improvement in the quality of the relationship between the amplitude of the signal which is processed and the lithological properties of the rocks, if a pair of vibratory signals is collected.

The explanation of this phenomenon seems to reside in the more efiective elimination which is obtained by means of two opposed pickup devices, of the nonuseful vibrations which arise from, for example, the transfer function.

In carrying out the invention the signal may be picked up at a measuring section, the location of which is chosen to be, according to the circumstances, either at the upper part of the drilling gear or at a place near the tool.

In a first mode of carrying out the method according to the invention, there is collected at the measuring section of the drilling gear at least one signal which is representative of the longitudinal vibrations of the said drilling gear, when using drilling tools comprising cutter wheels.

According to a preferred form of this first mode of carrying out the method according to the invention, there is collected at the upper part of the drilling gear, by means of accelerometers, at least one signal representing the longitudinal accelerations generated in the said gear by the operation of the drilling tool, the bearing faces of the accelerometers being horizontal, opposed to one another and offset axially.

In another variant of this form of the method, at least one signal representative of the stresses existing in the drilling gear is collected at the measuring section, by means of a plurality of strain gauges arranged parallel to the axis of the drilling gear and in a plane perpendicular to the said axis.

In a second mode of carrying out the method according to the invention, a signal representative of the torsional vibrations to which the said gear is subjected when using diamond drilling tools is collected at the upper part of the measuring section.

In a preferred form of this second mode of carrying out the method according to the invention, the torsional accelerations to which the drilling gear is subjected are collected at the measuring section, the signal being picked up by means of accelerometers which are arranged beneath the rotary table used for driving the drilling gear in rotation and the electrical axes of which are located in a plane perpendicular to the axis of the drilling gear.

According to a modification of this second mode of carrying out the method according to the invention, the torsional stresses to which the drilling gear is subjected are collected at a measuring section of the said gear, the pickup of the signal again being effected by means of strain gauges arranged beneath the rotary table by means of which the drilling gear is driven in rotation, the gauges being inclined at 45 to the drilling gear.

When the measuring section is located at the upper part of the drilling gear the longitudinal vibrations are picked up above the rotary table, while the torsional vibrations are picked up below the rotary table, the pickup devices being positioned in accordance with the nature of the vibrations which are to be collected, as will be apparent to anyone skilled in the art.

When the measuring section is disposed at a place near the tool, all signals are collected below the rotary table and it is only the difierent positioning of the pickup devices which determines the nature of the vibrations which are to be collected.

In the case in which the measuring section is placed at the upper part of the drilling gear, the collected signal is processed in an electronic unit situated near the point at which the signal is picked up and, after processing, it is correlated with the drilling depth.

In the second case the signal is treated at the bottom of the drill-hole and a signal representing the amplitude of the selected signal is transmitted to the surface either by way of the train of rods, using a suitable device such as a magnetostrictive bar or a piezoelectric crystal, or by means of the stream of mud, the pressure of which is modulated, by means of a valve for example.

The invention is also concerned with apparatus enabling the aforesaid methods to be carried into efi'ect, and comprising at least one vibration pickup device arranged on the measuring section of the drilling gear and fast with the latter, and delivering an electrical signal, means for processing the said signal in such a way as to limit its potential to two predetermined values, means for selecting a frequency band of the signal thus processed, centered on a frequency equal to the product of the frequency of rotation of the tool, times the number of operative elements of the latter, means for establishing from this selected signal fraction a value representing; the amplitude of this signal fraction and means for measuring this amplitude and correlating it with the depth at which the drilling tool is operating.

The apparatus used preferably comprises at least one pair of pickup devices and means for effecting the algebraic summation or the instantaneous differencing of the signals obtained from each pickup device, in order to obtain a single signal.

Since the measuring section can be placed at the upper part of the drilling gear or at a position near the tool, the forms of apparatus described below relate to these two modes of carrying out the invention but they are applicable more particularly to use with a pair of pickup devices. A similar system can be used when the signal is collected by a single pickup device, without departing from the scope of the present invention.

The measuring apparatus used at the upper part of the drilling gear will be described with reference to the following embodiments:

In a first embodiment, the devices for picking up longitudinal vibrations are constituted by two accelerometers disposed on opposite generatrices of a sleeve interposed between the injection head and the rod which drives the drilling gear in rotation, the accelerometers being placed on shoulders which are perpendicular to the axis of the sleeve and offset axially by from 2 centimeters to 9 meters, the electrical axes of the accelerometers being parallel and opposed.

In a second embodiment, the devices for picking up torsional vibrations are constituted by two accelerometers placed on diametrically opposed generatrices of the lower part of the rod which drives the drilling gear, the accelerometers being arranged on shoulders parallel to the axis of the driving rod and located in the axial plane, their electrical axes being parallel and opposed and located in a single plane perpendicular to the axis of the drilling gear.

relativeiy In a third embodiment, the devices for picking up longitudinal vibrations are strain gauges arranged on a sleeve interposed between the injection head and the rod for rotating the drilling gear, the pickups being placed at two diametrically opposed and axially offset points on the surface of the said sleeve.

In a fourth embodiment, the devices for picking up torsional vibrations are constituted by strain gauges arranged on the lower part of the rod by which the drilling gear is driven, the gauges being placed at two diametrically opposed points on the surface of the said sleeve, located in the same plane.

The first and third embodiments are preferably used when it is desired to observe the longitudinal vibrations, which permit of obtaining a useful signal when using tools operating by percussion, for example tools having cutter wheels, whereas the second and fourth embodiments are preferably used when it is desired to collect a useful signal when using tools such as diamond tools.

The apparatus which is used at a place near the drilling tool can be constituted in one of the following ways:

A first embodiment consists in placing an accelerometer which picks up longitudinal vibrations on a measuring connector disposed on the drilling gear at a place near the tool, the electrical axis of the said accelerometer being parallel to the axis of the drilling gear and this accelerometer supplying an electrical signal to a processing circuit which limits the potential of the signal, selects a frequency band, determines the amplitude of the said selected signal and uses this amplitude to control a device for transmitting this amplitude to a detector located at the upper part of the drilling gear, the said amplitude then being correlated with the drilling depth.

Three other embodiments using, respectively, strain gauges for measuring longitudinal vibrations, accelerometers for measuring torsional vibrations, and strain gauges for measuring torsional vibrations, can equally well be used with one pickup device or a pair of pickup devices.

The invention will be better understood from the following description, by way of example only and with reference to the accompanying drawings, of various embodiments of the said means for carrying into effect the method according to the invention. In the drawings:

FIG. 1 is a diagrammatic view of apparatus according to the invention mounted on a drilling installation.

FIG. 2 shows the details of the mounting of accelerometers when these are arranged on a sleeve interposed between the injection head and the rod by which the drilling gear is driven.

FIG. 3 is a diagram showing the mounting of stress pickup devices arranged on a sleeve interposed between the injection head and the square rod.

FIG. 4 is a circuit diagram of the electronic system of the arrangement which ensures the elimination of parasitic voltages due to shocks, in the case where accelerometers are used as pickup devices.

FIG. 5 is the circuit diagram of a filter used for selecting a frequency band when the frequency emitted by a cutter wheel tool rotating at a speed of 200 r.p.m. is picked up, and when the vibrations emitted by the external row of teeth of the cutter tool are collected.

FIG. 6 shows two diagraphs; the diagraph 73 is an acoustic diagraph obtained subsequently to drilling in the manner customarily in use hitherto, while the diagraph 74 is a diagraph obtained by the method according to the invention, concurrently with the drilling.

FIG. 7 shows an embodiment in which signals are collected at the bottom of a well or drill hole.

FIG. 8 shows a detail of the embodiment of FIG. 7.

FIG. 9 shows the detection circuit located at the upper part of the drilling gear for the purpose of detecting wavetrains emitted by the device of FIG. 8.

FIG. 10 shows a series of filters for adjacent frequencies, which are connected as a function of the operational speed of the drilling gear.

In FIG. 1, a drill derrick is represented at 1, the upper part 2 of the derrick carrying the stationary pulley assembly 3. The cable assembly connecting the stationary pulley assembly 3 to the block 5 carrying the movable pulley assembly is indicated at 4. Connected to the said block 5 is a hook 6, which supports the injection head 7. The upper part of this injection head 7 is fixed, while the lower part can be rotated by means of a bearing system. Indicated at 8 is the flexible injection pipe which is connected at one end to the injection head 7 and at the other end to the sludge pump assembly, not shown in the drawing. The rod by which the drilling gear is rotated is shown at 9. This rod is frequently of square formation and, in the remainder of the description, it will be referred to simply as the square rod." This rod 9 is driven in rotation by the rotating table 10, which itself is driven by a motor (not shown). A drill shaft is indicated diagrammatically at 11, while the drilling gear is shown at 12. This drilling gear is provided at its lower end with a drilling tool, indicated at 20. Interposed between the injection head 7 and the square rod 9 is a device 13 for measuring vibrations, which will be described in detail with reference to the following figures. The cable connecting the vibration measuring assembly 13 to the arrangement 15 which processes the electrical values representing the vibrations is indicated at 14. This signal processing assembly is connected, in the embodiment shown in the drawings, to a recording unit 16, the winding movement of the record carrier of which is controlled by a motor 19, which motor is connected by a line 18 to a pickup device 17 permitting the progress of the drilling to be measured. This measurement of the progress or advance of the drilling gives a measure of the variation in the level of the tool 20 in the drill hole 11 as a function of time.

FIG. 2 shows in greater detail the assembly 13 referred to in the foregoing description of the general arrangement. This assembly is made in the form of a sleeve which connects the injection head 7 to the square rod 9. The sleeve is represented at 21, this sleeve having a threaded female socket at its upper part and a male thread at its lower part. A member 210 bears on the fixed part of the injection head 7 (FIG. 1) and thus causes the external part 22 of the arrangement shown in FIG. 2 to be held in a fixed position. The sleeve 21 carries on its external surface, a shell 23 on which is fixed an insulating block 24, which is thus fast with the sleeve 21. This block 24 carries a series of metal rings, represented at 25a, 25b, 25c, 25d. Facing the block 24 and carried by the fixed part 22 is a second insulating block 26. This block 26 carries a series of brushes 27a, 27b, 27c, 27d which are adapted to slide on the rings 25a, 25b, 25c, 25d.

These brushes are connected to a series of electrical leads comprising a cable indicated at 28. The cable 28 extends out of the arrangement through a protective housing 30. Represented at 29 is a roller bearing carried by the sleeve 21 and there is also a stuffing box, the whole ensuring the fluidtightness of the chamber defined between the sleeve 21 and the external part 22. The fluid-tightness must be relatively good, so as to avoid fouling of the rings 25 and the brushes 27. Represented at 31 is a quartz accelerometer which delivers an electric signal under the influence of an acceleration. This accelerometer is rigidly mounted on a shoulder machined in the sleeve 21. This pickup 31 is connected by a cable 31a to an impedance coupling device 32. This impedance coupling device, which may be for example a field effect transistor having an input impedance of several megohms and an output impedance of the order of 1 k0, is connected on the one hand to the ring 25d by means of a measuring cable, while a second input which supplies the feed voltage of the transistor is connected by a second cable to another ring 25b. Indicated at 36 is a second accelerometer of the same type as the first, disposed on a generatrix of the sleeve diametrically opposite that on which the accelerometer 31 is positioned and at a distance of the order of a few tens of centimeters higher than this latter. The accelerometer 36 is likewise connected by a cable 360 to an impedance coupling device 37 having two outputs, one of which is connected to the ring 250 while the other is connected to the ring 25b. The connections are provided by the cables 38 and 39.

FIG. 3 shows another form of the vibration measuring assembly or pickup device indicated at 13 in the diagrammatic assembly shown in FIG. 1. Referring to FIG. 3, a sleeve 40 has a threaded female socket at its upper part and a male thread at its lower part. Indicated at 41 is a member which is fast with the sleeve 40 and which carries an insulating block 42. Represented at 43 is a jacket, which remains stationary by virtue of the fact that it is held fast, by means of a rod 44, with the upper part of the injection head. This jacket or chamber 43 thus remains stationary while the device described is being used.

The insulating block 42 carries a series of conducting rings 45a, 45b, 45c, 45d, these rings being connected by cables 46a, 46b, 46c, 46d to a series of stress gauges 47, 48, 49, 50. The gauges 47 and 48 are mounted vertically, whereas the gauges 49 and 50 which are mounted horizontally, that is to say perpendicular to the axis of the sleeve 40, serve as compensation gauges. The values recorded on the gauges 47 and 48 on the one hand, and on the gauges 49 and 50 on the other hand, are opposed to one another in a measuring bridge, taking account of the mechanical coefficients. Represented in FIG. 3 are brushes 51a, 51b, 51c, 51d, which slide on the rings 45a, 45b, 45c, 45d. These brushes, which are carried by an insulating block 52, are carried by the stationary jacket 43 and are connected to electrical leads 53a, 53b, 53c, 53d which are assembled to form a cable 54.

Two modifications of the arrangements described in connection with FIGS. 2 and 3 are possible. The vibration pickups, where these are accelerometers or stress gauges, can be positioned at some other point of the drilling gear, such that these pickups are situated beneath the rotary table at the time of drilling. Two longitudinal grooves are then formed in the square rod 9, permitting the passage of wires which connect the pickup devices to the collector system constituted by the rings and brushes. The accelerometers are mounted in recesses formed at the base of the square rod, so that the operative faces are parallel to the axis of the square rod and are fixed relatively to shoulders on the said rod, the operative faces of the two accelerometers being disposed in the same plane on either side of the axis of the square rod.

FIG. 4 shows the amplifier-filter assembly which effects the algebraic sum of the two signals and eliminates the parasitic component of the signal, which is due to shocks. In the case of FIG. 2, the signals provided by the accelerometers give signals which are out of phase by l; these are applied to the two inputs 56a and 56b of a differential amplifier 56 with a gain of about 20, which thus gives, in effect, the algebraic sum of the two signals. The differential amplifier 56 has a gain of 50,000 in open loop. Connected between the two inputs 56a and 56b of the differential amplifier 56 is a diode, represented at 57. Connected to the output 56c of the differential amplifier is a variable resistance 58, so that this resistance produces a feedback to the differential amplifier and brings the gain of the latter to a value close to 20.

Across the negative input 56a of the difi'erential amplifier and the output 560 is connected a series of capacitances 59a, 59b, 59c, forming a filter network which very strongly attenuates the signals which are beyond a predetermined frequency value; in a particular case, this value may be of the order of 5 kc./s., for example.

A number of pairs of diodes 60, 61 are connected between the terminals 560 and 560, so as to provide two series of diodes. The diodes 60 of the first series are connected so that conduction is allowed in the direction from 56a towards 56c, while the diodes of the second series are connected so that conduction is allowed in the direction from 560 towards 56a. The number of these diodes defines the threshold voltage of this system. With two pairs of diodes providing two series each containing two diodes, a peak-clipping threshold of the order of 1.2 volts is obtained, that is to say, a variable voltage of a maximum of i1 .2 volts is available between the reference line 7 62 and the output terminal 560. This makes it possible to eliminate the random signals of high amplitude which originate from phenomena foreign to the vibrations induced by the drilling tool.

FIG. illustrates a frequency selection arrangement. The output signal from across the terminals 56c and 62 of the system shown in FIG. 4 is received at the input 63. It is applied through two transistors 64 and 65, which serve as impedance adapters or couplers, to a total feedback differential amplifier 66, and then to a frequency selector device 67' formed by a series of capacitances, resistances and self-inductances. This filter is designed to act as a pass-band filter having a constant response coefficient in its narrow passband and an attenuation on either side of this band of about 50 decibels per octave. The filtered signal is applied to the input of a second differential amplifier 68, in the output circuit of which two diodes 69 and 70 are connected to be effective in opposite directions. These diodes rectify the alternations of the vibratory signal and the rectified signals are applied to the respective inputs of a third differential amplifier 71. The amplitudes of the positive and negative portions of the vibratory signal are thus added and, at the output 72, a signal is obtained which represents the maximum amplitude of the frequency band of the signal selected by the filter 67. This electrical value is available either for being recorded, or for use for the automatic control of the drilling. The signal is recorded or stored as a function of the depth at which the tool is working, using a pickup device by means of which it is possible to know the depth of the tool at any given time, for the purpose of controlling the advance of the recording medium or controlling the storage of the signal.

FIG. 6 represents at 73 a diagraph obtained by an acoustic method in a well drilled for the purpose of producing gas. Represented at 74 is a diagraph obtained by the method provided by the invention. The figures on the centerline of FIG. 6 represent the depths in meters at which are found the rocks whose properties are studied by the two methods. It will be seen that the general shape of the curves forming the two diagraphs is similar and that, in particular, the zones in which the speed of sound is high in the acoustic diagraph correspond to zones in which the amplitude of the vibratory signal observed by the method according to the invention has high values.

It must be pointed out that the mechanical diagraph 74 was obtained at the actual moment of drilling, while the acoustic diagraph 73 was obtained only after drilling was terminated.

The acoustic diagraph 73 represents the speed of sound in the rock. it is obtained by means of an ultrasonic transmitterreceiver system, which is displaced in the well, the depth at which the measurement is being made being known. A train of vibrations is transmitted by the transmitter and then received by the receiver. Measurement of the transit time enables the speed of sound in the rock to be determined by the relationship T=VL, where Tis the transit time,

V is the speed of sound, L is the transmitter-receiver distance.

On the other hand, in the diagraph 74, the amplitude of the curve represents the amplitude of the signal processed by the method according to the invention.

The similarity of the signals will be noted. In particular, at about 1,340 meters, two peaks coincide. Between 1,375 and 1,390 meters, there is coincidence between a series 'of signal peaks. Moreover, between 1,335 and 1,340, the same tendency is observed for the two signals.

This coincidence of tendency is found between 1,370 and 1,380 meters. Similarly, a tendency to decrease is found in the region of 1,390 meters.

It is thus seen that the measurement obtained by the method according to the invention is proportional to the speed of sound in the rock and this is correlated to the hardness of the rocks and their degree of compactness or density.

Similar correlations are obtained with the gamma-rayneutron diagraph or the rock density diagraph.

The operation of the apparatus provided by the invention and the use of the drilling method using a tool having cutter wheels will now be described as follows.

By means of the accelerometers 31 and 36 shown in FIG. 2, which are carried by the sleeve 21 and are located at 13 in FIG. 1, the accelerations resulting from longitudinal vibrations induced in the drilling gear by the operation of the cutter wheel tool are picked up. The voltages delivered by these accelerometers are processed by the impedance adapters 32 and 37. The low impedance voltage resulting therefrom is transmitted by the ring-brush system to the differential amplifier and voltage limiter assembly shown in FIG. 4. In this way, the components of the signal whose frequency is higher than 5 kc./s. are eliminated and also the amplitudes higher than about 1.2 volts.

The signal leaving the arrangement shown in FIG. 4 is applied to the input of the filter shown in FIG. 5, which ensures the filtering in a pass band which is between 40 and c./s. This band is centered on the frequency of 70 c./s., corresponding to a speed of rotation of the tool of 210 rpm. which produces an excitation frequency of the drilling gear of 70 c./s. In fact, with each rotation of the drilling gear, 20 elementary pulses are transmitted by the external row of teeth on each cutter. It was found that this transmission of pulses, accounted for by the external row of teeth, predominated over the transmissions accounted for by the cutters or by the teeth forming the intermediate row carried by each cutter. However, it is possible, by'using a different pass-band filter, to analyze the vibrations generated by the cutters or by the intermediate row of teeth. It is also possible to control an adjustable pass-band filter by means of a signal obtained from the frequency of rotation of the drilling gear, for example from the instantaneous speed of rotation.

The amplitude of the filtered signal is recorded as a function of the advance of the tool while the latter is working at the cutting position.

This signal can be used as an input value in an arrangement by means of which it is possible to establish, from this signal, control values serving for the automatic control of the drilling by acting on the brake of the winch of the drilling gear, so as to increase or decrease the weight bearing on the tool, and on the power supply to the motor, so as to vary the speed of rotation and/or the rate of delivery of the mud. The transmission of the signals between the pickup devices and the processing circuits is effected, in the embodiments described above, by means of wired connections. It would be possible instead to achieve this connection by means of Herzian (radio) waves or by means of acoustic waves, for example ultrasonic waves.

In FIG. 7 a drilling derrick is shown at 101, a suspension cable assembly which supports a train of drilling rods 108, being shown at 102. At 103 is shown an injection head which pennits mud to be introduced into the drilling rods and a detection connector 104 receives information by way of the stream of mud, processes this information and transmits it to a memory store. The rod which drives the drilling gear is shown at 105 while the rotary table is shown at 106. At 107 is shown the ground formation in which a well or drill hole has been bored by means of the train of rods 108, which supports a train of boring rods 109. In the train of rods 109 there is incorporated a special measuring rod 110 serving for the transmission of signals from the bottom of the drill hole to the surface and which constitutes a part of the device according to the invention.

To this special rod 110 there is connected a tool-carrying device 111 fitted with a tool 112 which directly attacks the rock. A receiver 113 is disposed at a certain distance from the drill hole to receive information passing by way of the connector 104 and permits one to obtain, as a function of the depth, a magnitude which is characteristic of the mechanical properties of the rocks, which magnitude can either be recorded or can be used for the automatic control of drilling.

In order to achieve this transmission, the connector 104 is provided with a radio transmitter having an antenna 114. The device 113 has a receiving antenna 115.

FIG. 8 shows the details of the rod 110 referred to in the description of HO. 7. It comprises a body 116. Inside this body there is disposed firstly an assembly 117 for modulating the pressure of the mud, constituted by a valve, the opening and closing of which are controlled sequentially by a circuit unit 118 receiving control signals from an electronic circuit 119 situated in the lower part of the body. The valve member 1 l7 closes against a seat 120 through which the stream of mud normally passes, thereby producing pressure pulses. Signals coming from the electronic circuit 119 are transmitted to the pressure modulator by a connection 121. Between the pressure modulator and the electronic circuit 1 19 there is disposed the measuring connector 122 which is a rigid steel connector on which are mounted series of strain gauges 123 and 124 and/or acceleration pickups 125, 126 and 127. This connector is protected from the external medium by the jacket 122a which is fixed at one of its ends and free at the other, fluidtightness being provided at this other end by means of a toroidal washer.

The various pickups are connected by cables which pass through a tube 128 which connects the chamber defined between the connector and the jacket to the electronic assembly 1 19.

The stream of mud, after passing through the space between the valve member 117 and the seat 121), flows around the modulation device 118 and passes into the interior 129 of the measuring connector. A recess 130 allows the stream of mud to pass into the annular space 131 surrounding the electronic assembly 119. Recesses 132 allow the current of mud to pass back into the interior of the tool-carrier 133 by way of the tube 134. Meanwhile the current of mud is used to drive a turbine 135 which supplies the electrical energy necessary for the operation of the electronic assembly 119.

Acceleration detectors 126 are placed on the two opposite generatrices of the connector in such a way that their axis is parallel to the axis of the connector.

The acceleration detectors 125 are arranged on the opposite generatrices at the same height, their axis being perpendicular to the axis of the connector.

The detectors 125 permit torsional vibrations to be selected while the detectors 126 permit longitudinal vibrations to be selected.

The detector 127 is arranged parallel to the detector 125, this single detector permitting a sinusoidal oscillation to be obtained whose period is directly related to the speed of rotation. This detector enables the basic frequency to be defined, upon a multiple of which basic frequency the filtering of the vibrations is centered. The frequency upon which the frequency is centered is a multiple of the speed of rotation.

The gauges 123 and 124 permit either the longitudinal vibrations or the torsional vibrations to be selected. For this purpose the gauges are arrangedin a half-bridge in a direction which is related to the type of vibration which one wishes to measure.

Although the assemblies of acceleration detectors and deformation gauges have been shown in the same figure, one of these assemblies can be used on its own in order to select one or the other mode of vibrations, according to whichever may appear more representative.

The processing of the electrical values supplied by the gauges or acceleration detectors is effected in the manner which will be described below.

When acceleration detectors are used they are disposed on two opposite generatrices of the measuring connector and the electrical signals supplied by these detectors are opposed to one another in a differential amplifier. in this way the signals representing the vibratory state which is being investigated are added while all the signals representing parasitic vibrations are eliminated. At the output of the differential amplifier there is available a single signal whose amplitude is substantially double the effective signal supplied by one of the detectors. This signal is then processed. in a first stage the amplitude is limited to a value which is determined in advance; this can be done in a saturation amplifier whose maximum amplitude is. detertreated is applied to a band-pass filter whose mean frequency is a multiple of the speed of rotation.

For this purpose the accelerometer 127 delivers a sinusoidal potential which can be selectively amplified in the band from 0.2 to 5 Hz. Then by means of a frequency multiplier, one multiplies the frequency thus obtained by a number which takes account of the number of attacking elements of the tool. For example, when one uses the preponderant mode of the vibrations delivered by the outer row of teeth carried by the wheels of a tricone tool, the multiplication factor is about 20.

The circuit described above is shown in FIG. 9 in which the acceleration detectors 136, 137 are shown connected by leads 138, 139 to a differential amplifier 140. The output 141 of the said differential amplifier is connected to a peak-clipping device 142 whose output 143 is connected to a band-pass filter 144 controlled by a frequency which is a multiple of the speed 'of rotation measured by the accelerometer 127 in FIG. 8. The sinusoidal potential supplied by this accelerometer 127 is filtered by a filter 145, then the frequency is multiplied by the frequency multiplier 146.

The signals supplied by strain gauges are processed in a similar manner. The signal is obtained directly due to the arrangement of the deformation gauges in the form of a whole bridge, the compensation gauges being arranged to measure the vibrations being investigated and to eliminate the effects of parasitic vibrations and of temperature and pressure.

The filtering of the signal coming from the pickups after processing can be effected by a filter controlled by the speed of rotation of the drilling gear.

In a modification which is applicable whatever the position of the measuring section, a series of filters having a fixed passband and a fixed mean frequency can be used. The signal obtained from the pickups is supplied to the filter whose mean frequency corresponds to the desired frequency of filtering.

This is shown in FIG. 10. The signal which gives a measure of the speed of rotation is obtained from the pickup 127. It is filtered by the fixed filter constituted by the inductance 147 and the capacitance 148. The filtered signal is applied to a selector 149 which commutates the input 150 to various outputs 151, 152, 153, 154 each of which is connected to a bandpass filter 155, 156, 157, 158.

The central frequency of the various filters is different. The frequencies are distributed in such a way that the upper cutoff frequency of each filter is substantially equal to the lower cutoff frequency of the following filter. The frequency of commutation is related to the frequency of the filters.

The signals from the various filters are collected by a single output element 159 and the resulting signal is coded and then transmitted to the device which modulates the pressure of the mud.

in the case where the measuring section is in the vicinity of the tool and the signal representing the amplitude is transmitted by modulating the pressure of the stream of mud, the pressure variations are detected by a pressure detector disposed inside the connector 104 described with reference to FIG. 7. This detector influences the modulating action of a transmitter of electronic waves which is arranged in the same connector 104. The resulting transmission is received by the device 113 which, after appropriate processing, supplies an electrical value which can either be recorded or can be used as a control value for controlling the input of a computer which controls drilling.

lt is within the scope of the invention to replace the device for modulating the pressure of the mud by a magnetostrictive transmitter coupled to the train of rods. In this case the coded signal is used either for direct control or to control the modulation of the magnetostrictive transmitter. A receiver of the same kind, that is to say a magnetostrictive receiver for example, is arranged in the connector which is situated above the drive rod. It enables the signals transmitted by the train of rods to be detected and applied to the l-lerzian transmitter associated therewith. The signal which is thus transmitted to the processing apparatus is transformed into a value which can either be recorded or used for controlling drilling.

Another mode of carrying the invention into effect consists in using only a single detector, for example a single accelerometer or a single pair of strain gauges (one operative and the other serving for compensation) or a pressure detector which is responsive to variations in the pressure of the mud.

in this case the differential amplifier is replaced by an ordinary amplifier connected with a frequency filter and a level limiter. The other parts of the measuring circuit are unchanged.

It will be apparent that the invention can be carried into effect using modifications of the system described but based on the same basic principle, without departing from the scope of the invention as defined by the appended claims.

We claim:

1. A method for measuring characteristics of rocks during drilling by means of drilling gear comprising a drilling tool having at least one set of elements for attacking rock at a depth in a drill hole, means for driving said drilling tool at a certain frequency of rotation, and a measuring section for monitoring the drilling operation, said method comprising the steps of generating signals which are representative of vibrations of said drilling gear detected at at least one point on said measuring section, eliminating from said signals parasitic voltages outside a range defined by two predetermined values of opposite polarities, so as to provide a resultant signal, selecting from said resultant signal a signal comprising a frequency band centered on a frequency equal to said frequency of rotation of said tool multiplied by the number of said attacking elements in at least one of said sets, which selected signal has an amplitude within said band which is directly related to lithological properties of rocks being attacked by said tool, measuring said amplitude, and correlating it with the depth at which said tool is working.

2. A method according to claim 1, in which vibrations are detected at at least one pair of points located on said drilling gear, said method further comprising the step of effecting algebraic summation of said generated signals.

3 A method according to claim 2, in which the vibrations detected at said pair of points are detected by means of two pickup devices arranged at points on two diametrically opposite generatrices of said measuring section.

4. A method according to claim 3, in which said generated signals are representative of longitudinal vibrations, and in which said diametrically opposite points at which said pickup devices are located are offset relatively to one another along the axis of said drilling gear by a distance which is between 2 centimeters and 9 meters.

5. A method according to claim 1, in which said selected signal is obtained by selecting signal components in said frequency band by means of a band-pass filter arrangement having a mean frequency which is controlled as a function of the instantaneous speed of rotation of said drilling gear multiplied by the number of attacking elements in at least one set.

6. A method according to claim 1, in which said measuring section is situated at the upper part of said drilling gear.

7. A method according to claim 1, in which said measuring section is situated near said tool.

8. A method according to claim 1, utilizing a tool having a number of cutter wheels, each having inner and outer rows of cutting teeth, in which said generated signals are representative of longitudinal vibrations, and said selected signal is selected in a frequency band centered on a frequency equal to said frequency of rotation of said tool multiplied by the number of said cutter wheels or the number of cutting teeth in said outer row of each of said cutter wheels.

9. A method according to claim 8, in which said signals representing longitudinal vibrations are generated by means of at least one pair of accelerometers which are offset axially and have electrical axes which are parallel to the axis of said drilling gear.

10. A method according to claim 8, in which said signals represent longitudinal stresses in said drilling gear and are generated by means of at least one pair of strain gauges disposed parallel to the axis of said drilling gear.

11. A method according to claim 1, in which said tool is a diamond tool having a plurality of diamond-bearing surfaces and in which said generated signals represent torsional vibrations, said selected signal being selected in a frequency band centered on a frequency which is equal to the frequency of rotation of said tool multiplied by the number of said diamond-bearing surfaces.

12. A method according to claim 11, in which said generated signals represent torsional accelerations which are detected by means of accelerometers having their electrical axes located in a plane perpendicular to the axis of said drilling gear.

13. A method according to claim 11, in which said generated signals represent torsional stresses which are detected by means of strain gauges located in a plane which is inclined at 45 to the axis of said drilling gear or of said measuring section.

14. In combination with drilling gear comprising a drilling tool having at least one set of elements for attacking rock within a drill hole, and means for driving said drilling tool at a selected frequency of rotation, the improved apparatus for measuring rock characteristics during drilling which comprises:

vibration responsive means including at least one vibration pickup device positioned to detect vibrations by said tool, said vibration responsive means being adapted to produce at its output an electric signal representative of said vibrations, voltage-limiting means connected to the output of said vibration responsive means for eliminating from said signal voltages outside a predetermined range of values,

means connected to the output of said voltage-limiting means for selecting from said signal a frequency band centered on a frequency equal to the frequency of rotation of said tool multiplied by the number of attacking elements in at least one of said sets,

and means responsive to the amplitude of said signal in said frequency band for correlating said amplitude with the depth in said drill hole at which said drilling tool is operatmg.

15. The combination according to claim 14, comprising at least two vibration pickup devices and means for effecting the algebraic summation of signals delivered by the respective pickup devices so as to supply a single signal to said voltagelimiting means.

16. The combination according to claim 13, in which said pickup devices are constituted by two accelerometers for picking up longitudinal accelerations, disposed on two opposed generatrices of a connector situated at the lower part of said drilling gear near said tool, said accelerometers being arranged on shoulders perpendicular to the axis of said connector and being rigidly attached to said shoulders.

17. The combination according to claim 42, in which said pickup devices are constituted by two accelerometers for picking up torsional vibrations, disposed diametrically opposite one another on two generatrices of a connector which is inserted in the drilling gear near said drilling tool, said accelerometers being disposed on shoulders parallel to the axis of said connector and the electrical axes of said accelerometers being parallel and diametrically opposite each other.

18. The combination according to claim 15, in which said pickup devices are constituted by strain gauges for picking up longitudinal vibrations, disposed on a connector located at the lower part of said drilling gear near said tool, said gauges being disposed at two diametrically opposite points on the longitudinal surface of said connector and the axes of said gauges being parallel to the axis of said measuring section.

19. The combination according to claim 15, in which said pickup devices are constituted by strain gauges for picking up torsional vibrations, disposed on a connector located at the lower part of said drilling gear near said tool, said gauges being disposed at two diametrically opposite points on the longitudinal surface of said connector, which lie in a common plane inclined at 45 to the axis of said connector.

20. The combination according to claim 15, in which said vibration-responsive means is situated at the upper part of said drilling gear.

21. The combination according to claim 20, in which said driving means comprises rod means and said drilling gear includes a drilling head and a sleeve interposed between said head and said rod means, and said pickup devices are accelerometers for detecting longitudinal vibrations, disposed on two opposite generatrices of said sleeve, on shoulders which are perpendicular to the axis of said sleeve, and offset along said axis, said accelerometers being rigidly connected to said shoulders.

22. The combination according to claim 20, in which said drive means comprises rod means and said pickup devices comprise two accelerometers responsive to torsional vibrations, disposed on two diametrically opposite generatrices of the lower part of said rod means, said accelerometers being rigidly mounted in a common plane, on shoulders which are parallel to the axis of said rod means and are situated in an axial plane.

23. The combination according to claim 20, in which said drive means comprises rod means and said drilling gear includes a drilling head and a sleeve interposed between said head and said rod means, and said pickup devices comprise strain gauges responsive to longitudinal vibrations, disposed on two diametrically opposite generatrices of said sleeve at two points slightly offset axially of said sleeve.

24. The combination according to claim 20, in which said drive means comprises rod means and said pickup devices comprise strain gauges responsive to torsional vibrations, said gauges being disposed at two diametrically opposite points at the lower part of the longitudinal surface of said rod means, in a common plane which is inclined at 45 to the axis of said drilling gear.

25. The combination according to claim 14, in which said vibration responsive, voltage limiting and frequency bandselecting means are situated at the lower part of said drilling gear, and which comprises means for transmitting to the surface the amplitude of said selected signal.

26. The combination according to claim 25, in which said means for transmitting the amplitude of said selected signal to the surface comprises rod means through which said drilling tool is driven, together with magnetostrictive means for transmitting along said rod a signal responsive to the amplitude of said selected signal, and a magnetostrictive receiver above the ground for detecting the signal transmitted along said rod and transmitting it to said correlating means.

27. The combination according to claim 48, in which said means for transmitting the amplitude of said selected signal comprise conduit means for the passage of a stream of mud, a valve controlling the pressure of said stream of mud in said conduit means, and means for operating said valve in dependence upon said selected signal, whereby the pressure of said stream of mud is modulated in accordance with the amplitude of said selected signal.

UNITED STATES PATENT OFFICE CERHFMATE 6F CORRECTION Patent No. 3 82 Dated December 7 s 1971 CLAUDE JEAN QUICHAUD MICHEL H RAYNAUD and JEAN LUTZ Inventor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

['32] Priorities Oct. 30, 1968 [33] France Dec. 11, 1968, France, No. 1775 8; Feb. 27, 1969, France, No, 69051 42 Signed and sealed this Lrth day of July V2729 (SEAL) Attest:

I EDWARD 1-1.,FLJLZICMEI- JR.a ROBERT GOTTSCHAL K Attesting Officer Co misffioner of Patents FORM PC4050 0459) I USCOMM-DC 60376-P69 A U.S. GOVERNMENT PRINTING OFFICE I959 O366334 

1. A method for measuring characteristics of rocks during drilling by means of drilling gear comprising a drilling tool having at least one set of elements for attacking rock at a depth in a drill hole, means for driving said drilling tool at a certain frequency of rotation, and a measuring section for monitoring the drilling operation, said method comprising the steps of generating signals which are representative of vibrations of said drilling gear detected at at least one point on said measuring section, eliminating from said signals parasitic voltages outside a range defined by two predetermined values of opposite polarities, so as to provide a resultant signal, selecting from said resultant signal a signal comprising a frequency band centered on a frequency equal to said frequency of rotation of said tool multiplied by the number of said attacking elements in at least one of said sets, which selected signal has an amplitude within said band which is directly related to lithological properties of rocks being attacked by said tool, measuring said amplitude, and correlating it with the depth at which said tool is working.
 2. A method according to claim 1, in which vibrations are detected at at least one pair of points located on said drilling gear, said method further comprising the step of effecting algebraic summation of said generated signals.
 3. A method according to claim 2, in which the vibrations detected at said pair of points are detected by means of two pickup devices arranged at points on two diametrically opposite generatrices of said measuring section.
 4. A method according to claim 3, in which said generated signals are representative of longitudinal vibrations, and in which said diametrically opposite points at which said pickup devices are located are offset relatively to one another along the axis of said drilling gear by a distance which is between 2 centimeters and 9 meters.
 5. A method according to claim 1, in which said selected signal is obtained by selecting signal components in said frequency band by means of a band-pass filter arrangement having a mean frequency which is controlled as a function of the instantaneous speed of rotation of said drilling gear multiplied by the number of attacking elements in at least one set.
 6. A method according to claim 1, in which said measuring section is situated at the upper part of said drilling gear.
 7. A method according to claim 1, in which said measuring section is situated near said tool.
 8. A method according to claim 1, utilizing a tool having a number of cutter wheels, each having inner and outer rows of cutting teeth, in which said generated signals are representative of longitudinal vibrations, and said selected signal is selected in a frequency band centered on a frequency equal to said frequency of rotation of said tool multiplied by the number of said cutter wheels or The number of cutting teeth in said outer row of each of said cutter wheels.
 9. A method according to claim 8, in which said signals representing longitudinal vibrations are generated by means of at least one pair of accelerometers which are offset axially and have electrical axes which are parallel to the axis of said drilling gear.
 10. A method according to claim 8, in which said signals represent longitudinal stresses in said drilling gear and are generated by means of at least one pair of strain gauges disposed parallel to the axis of said drilling gear.
 11. A method according to claim 1, in which said tool is a diamond tool having a plurality of diamond-bearing surfaces and in which said generated signals represent torsional vibrations, said selected signal being selected in a frequency band centered on a frequency which is equal to the frequency of rotation of said tool multiplied by the number of said diamond-bearing surfaces.
 12. A method according to claim 11, in which said generated signals represent torsional accelerations which are detected by means of accelerometers having their electrical axes located in a plane perpendicular to the axis of said drilling gear.
 13. A method according to claim 11, in which said generated signals represent torsional stresses which are detected by means of strain gauges located in a plane which is inclined at 45* to the axis of said drilling gear or of said measuring section.
 14. In combination with drilling gear comprising a drilling tool having at least one set of elements for attacking rock within a drill hole, and means for driving said drilling tool at a selected frequency of rotation, the improved apparatus for measuring rock characteristics during drilling which comprises: vibration responsive means including at least one vibration pickup device positioned to detect vibrations by said tool, said vibration responsive means being adapted to produce at its output an electric signal representative of said vibrations, voltage-limiting means connected to the output of said vibration responsive means for eliminating from said signal voltages outside a predetermined range of values, means connected to the output of said voltage-limiting means for selecting from said signal a frequency band centered on a frequency equal to the frequency of rotation of said tool multiplied by the number of attacking elements in at least one of said sets, and means responsive to the amplitude of said signal in said frequency band for correlating said amplitude with the depth in said drill hole at which said drilling tool is operating.
 15. The combination according to claim 14, comprising at least two vibration pickup devices and means for effecting the algebraic summation of signals delivered by the respective pickup devices so as to supply a single signal to said voltage-limiting means.
 16. The combination according to claim 13, in which said pickup devices are constituted by two accelerometers for picking up longitudinal accelerations, disposed on two opposed generatrices of a connector situated at the lower part of said drilling gear near said tool, said accelerometers being arranged on shoulders perpendicular to the axis of said connector and being rigidly attached to said shoulders.
 17. The combination according to claim 42, in which said pickup devices are constituted by two accelerometers for picking up torsional vibrations, disposed diametrically opposite one another on two generatrices of a connector which is inserted in the drilling gear near said drilling tool, said accelerometers being disposed on shoulders parallel to the axis of said connector and the electrical axes of said accelerometers being parallel and diametrically opposite each other.
 18. The combination according to claim 15, in which said pickup devices are constituted by strain gauges for picking up longitudinal vibrations, disposed on a connector located at the lower part of said drilling gear near said tool, said gaUges being disposed at two diametrically opposite points on the longitudinal surface of said connector and the axes of said gauges being parallel to the axis of said measuring section.
 19. The combination according to claim 15, in which said pickup devices are constituted by strain gauges for picking up torsional vibrations, disposed on a connector located at the lower part of said drilling gear near said tool, said gauges being disposed at two diametrically opposite points on the longitudinal surface of said connector, which lie in a common plane inclined at 45* to the axis of said connector.
 20. The combination according to claim 15, in which said vibration-responsive means is situated at the upper part of said drilling gear.
 21. The combination according to claim 20, in which said driving means comprises rod means and said drilling gear includes a drilling head and a sleeve interposed between said head and said rod means, and said pickup devices are accelerometers for detecting longitudinal vibrations, disposed on two opposite generatrices of said sleeve, on shoulders which are perpendicular to the axis of said sleeve, and offset along said axis, said accelerometers being rigidly connected to said shoulders.
 22. The combination according to claim 20, in which said drive means comprises rod means and said pickup devices comprise two accelerometers responsive to torsional vibrations, disposed on two diametrically opposite generatrices of the lower part of said rod means, said accelerometers being rigidly mounted in a common plane, on shoulders which are parallel to the axis of said rod means and are situated in an axial plane.
 23. The combination according to claim 20, in which said drive means comprises rod means and said drilling gear includes a drilling head and a sleeve interposed between said head and said rod means, and said pickup devices comprise strain gauges responsive to longitudinal vibrations, disposed on two diametrically opposite generatrices of said sleeve at two points slightly offset axially of said sleeve.
 24. The combination according to claim 20, in which said drive means comprises rod means and said pickup devices comprise strain gauges responsive to torsional vibrations, said gauges being disposed at two diametrically opposite points at the lower part of the longitudinal surface of said rod means, in a common plane which is inclined at 45* to the axis of said drilling gear.
 25. The combination according to claim 14, in which said vibration responsive, voltage limiting and frequency band-selecting means are situated at the lower part of said drilling gear, and which comprises means for transmitting to the surface the amplitude of said selected signal.
 26. The combination according to claim 25, in which said means for transmitting the amplitude of said selected signal to the surface comprises rod means through which said drilling tool is driven, together with magnetostrictive means for transmitting along said rod a signal responsive to the amplitude of said selected signal, and a magnetostrictive receiver above the ground for detecting the signal transmitted along said rod and transmitting it to said correlating means.
 27. The combination according to claim 48, in which said means for transmitting the amplitude of said selected signal comprise conduit means for the passage of a stream of mud, a valve controlling the pressure of said stream of mud in said conduit means, and means for operating said valve in dependence upon said selected signal, whereby the pressure of said stream of mud is modulated in accordance with the amplitude of said selected signal. 